1. Field of the Invention
The present invention relates to the field of semiconductor fabrication and processing and more particularly to the field of the electroless deposition of metal alloy barrier layers over copper.
2. Discussion of Related Art
Copper presents different engineering challenges during different stages of the semiconductor fabrication process. One such challenge is the formation of copper and tin intermetallic compounds on the interface between copper bumps on a die and the tin-based solder used to electrically connect that die to a substrate package, such as a ball grid array (BGA) or pin grid array (PGA), in a flip-chip process. The flip-chip process is a method to electrically connect the die to the package carrier. The package carrier, either a substrate or a leadframe, then provides the connection from the die to the exterior of the package. The interconnection between the die and the carrier in flip chip packaging is made through a conductive “bump” that is placed directly on the die surface. That bump may be formed of copper. The bumped die is then flipped over and aligned with tin-based solder bumps formed on the package carrier. The copper bumped die is attached to the package carrier through a metallic bond formed between the copper bumps on the die and the tin-based solder bumps on the package carrier. This metallic bond is formed by reflowing the tin-based solder to attach the solder to the copper bumps on the die. The direct contact of the copper of the copper bumps with the tin-based solder causes the diffusion of the copper from the copper bumps and the tin from the solder to form copper-tin intermetallic compounds. The diffusion of the copper and of the tin creates voids within the connection between the die and the package carrier. The voids cause shorts, excessive heat, and eventually failure of the device. Cobalt based barrier layers, such as CoBP and CoP, have been formed over the copper bumps to prevent the diffusion of the copper and tin and thus the formation of the intermetallic compounds and voids. But, these barrier layers do not effectively block the diffusion of copper and tin. These refractory metal based barrier layers also became very brittle under the stresses placed on the package during fabrication and during the use of the device. Once the barrier layer material becomes brittle and breaks apart, there is further free diffusion of copper and tin through the cracks in the barrier layer and the formation of intermetallic compounds.
Another challenge of copper is the electromigration of copper from the interconnect lines and the vias of an integrated circuit into the surrounding dielectric material, particularly when the surrounding dielectric material is a porous low-k dielectric material. The diffusion of copper often requires the use of a diffusion barrier encapsulating or capping the copper interconnects or vias. A variety of materials are known for forming diffusion barriers on copper. Such materials include CoBP or CoWP alloys. However, these materials may be oxidized, removed, or corroded during the etching or cleaning of vias or trenches. Also, copper may migrate through the cobalt containing materials into the surrounding dielectrics causing shorts, or oxygen may migrate into the copper through the cobalt based material causing oxidation of the copper interconnects.